Function qualify_join_schema_sides 

fn qualify_join_schema_sides(
    join_schema: &DFSchema,
    left: &LogicalPlan,
    right: &LogicalPlan,
) -> Result<DFSchema>

Qualifies the fields in a join schema with “left” and “right” qualifiers without mutating the original schema. This function should only be used when the join inputs have already been requalified earlier in try_new_with_project_input.

The purpose is to avoid ambiguity errors later in planning (e.g., in nullability or data type resolution) when converting expressions to fields.